I assume that you're using manual cross-hair marks rather than actually using the ARMS and Graphtec type 1 or 2 marks. If you were using the ARMS then this wouldn't be a question.
If that is the case, why don't you use the plotter's built-in light pen to locate the marks? I assume, since you have the ARMS, that your machine has the standard Graphtec light pen as a pen selection. You might consider reading whatever passes for a manual that came with the machine.
If you don't have a light pen, then you can either buy an optical bomb sight, very pricey, a light pen, cheaper, or, if you have really good eyes, just use the blade itself. With the bomb sight and the light pen, you replace the blade holder with either of these, set the marks, and then re-install the blade holder. A pain in the ass.